Lower bainite has a microstructure and crystallographic features which are very similar to those of upper bainite. The major distinction is that cementite particles also precipitate inside the plates of ferrite Figure. There are, therefore, two kinds of cementite precipitates: those which grow from the carbon-enriched austenite which separates the platelets of bainitic ferrite, and others which appear to precipitate from supersaturated ferrite. These latter particles exhibit the "tempering" orientation relationship which is found when carbides precipitate during the heat treatment of martensite, often described as the Bagaryatski orientation relationship:
[0 0 1] Fe3C || [ -1 0 1]alpha
[1 0 0] Fe3C || [ 1 1 1]alpha
[0 1 0] Fe3C || [ -1 2 -1]alpha
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The microstructure of lower bainite. Notice the precipitation of several variants of carbide particles within the plate of lower bainitic ferrite. Lower bainite otherwise also consists of fine platelets organised in sheaves, with each platelet separated partially by films of carbon-enriched retained austenite or carbides. After Bhadeshia and Edmonds, Metallurgical Transactions A, volume 10A (1979) 895-907. |
The carbides in the ferrite need not always be cementite. Depending on the chemical composition and the transformation temperature, other transition carbides may precipitate first. For example, in high-carbon steels containing more than about 1 wt.% silicon (which retards cementite formation), epsilon carbide is commonly observed to precipitate in the bainitic ferrite.
In contrast to tempered martensite, the cementite particles in lower bainite frequently precipitate in just one variant of the orientation relationship ( Figure), such that they form parallel arrays at about 60 ° to the axis of the bainite plate. In tempered martensite, the carbides tend to precipitate in Widmanstatten arrays. This peculiar mode of precipitation in lower bainitic ferrite may arise because the carbides nucleate at the ferrite/austenite interface, and hence attempt to adopt a unique variant of the orientation relationship, one which gives an optimum match to both the austenite and ferrite with which they are in contact.
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A transmission electron micrograph of lower bainite showing a single variant of carbide particles in each plate. Single variants tend to form when the driving force for cementite precipitation is small, i.e. in low carbon steels or at high temperatures where the carbon can escape rapidly from supersaturated ferrite. After Bhadeshia, Acta Metallurgica, volume 28 (1980) 1103-1114. |
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Another plausible explanation is that the carbide precipitation is influenced by the stresses associated with the displacive growth of lower bainite. The effect would be less pronounced during the tempering of martensite because the driving force for carbide precipitation is larger.
The carbides in the lower bainite are extremely fine, just a few nanometres thick and about 500 nm long. Because they precipitate within the ferrite, a smaller amount of carbon is partitioned into the residual austenite. This in turn means that fewer and finer cementite particles precipitate between the ferrite plates, when compared with an upper bainitic microstructure. An important consequence is that lower bainite is usually found to be much tougher than upper bainite, in spite of the fact that it also tends to be stronger. The coarse cementite particles in upper bainite are notorious in their ability to nucleate cleavage cracks and voids.